The present invention relates to a novel coating composition used for forming a magnetic layer on a magnetic recording medium or, in particular, to a novel resin used as a binder ingredient in the coating composition capable of uniformly dispersing and suitable for binding particles of ferromagnetic materials.
Magnetic recording media such as magnetic recording tapes and the like usually have a structure that a base film or sheet such as polyester films is provided on one surface with a magnetic coating layer composed of particles of a ferromagnetic material and a resinous ingredient as a binder of the particles or a matrix of the layer. Various kinds of magnetic materials are used for the purpose including iron oxides of the formulas .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 undoped or doped with cobalt ions adsorbed on the particles thereof, and chromium oxide of the formula CrO.sub.2 as well as powders of metals such as iron and cobalt and alloys such as those of iron and cobalt optionally containing nickel having a needle-like particle configuration. Along with the progress in the technology of magnetic signal recording in recent years for video tapes and audio tapes toward a higher recording density and a higher output in the playing of the tapes prepared by short-wave recording, these magnetic powders are also required to have a finer and finer particle size distribution. One of the difficult problems in the preparation of a coating composition for magnetic recording media is that, due to the very large magnetic moment of such fine magnetic particles, the particles sometimes form agglomerates so that a great difficulty is encountered in obtaining the desirable uniform distribution of the individual magnetic particles in the matrix resin.
With an object to solve the above mentioned technical problem, extensive investigations have been undertaken to improve the affinity of the matrix resin with the magnetic particles. For example, a proposal has been made to use a vinyl chloride-based copolymeric resin having polar groups such as carboxyl and hydroxy groups introduced into the molecular structure. The resins of this type are still not quite satisfactory in respect of the dispersibility of ferromagnetic particles therein so that the magnetic recording media prepared with the resin as a binder of the ferromagnetic particles are inferior in the magnetic properties such as the residual magnetic flux and squareness ratio of the magnetic hysteresis loop and sometimes cause falling of the magnetic particles to have a greatly decreased durability in service. See, for example, Japanese Patent Publication Nos. 55-34493, 55-35768 and 56-12934 for the carboxyl-containing resins and Japanese Patent Publication Nos. 55-28130, 55-43189 and 55-35770 for the hydroxy-containing resins.